The present invention is related to orthopedic implants and is more particularly related to intervertebral implants.
The human spinal column has more than twenty discrete bones sequentially coupled to one another by a tri-joint complex that consists of an anterior disc and two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. The more than twenty bones are anatomically categorized in one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine extends from the base of the skull and includes the first seven vertebrae. The intermediate twelve vertebrae make up the thoracic portion of the spine. The lower portion of the spine comprises five lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis. While the sacral region is an integral part of the spine, for the purposes of fusion surgeries and for this disclosure, the word spine shall refer only to the cervical, thoracic, and lumbar regions.
The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in dose proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.
The intervertebral disc disposed between the vertebrae in the human spine has a peripheral fibrous shroud (the annulus) that surrounds a spheroid of flexibly deformable material (the nucleus). The nucleus comprises a hydrophilic, elastomeric cartilaginous substance that cushions and supports the separation between the bones. The nucleus also permits articulation of adjacent vertebral bones relative to one another to the extent such articulation is allowed by the other soft tissue and bony structures surrounding the disc. The additional bony structures that define pathways of motion in various modes include the posterior joints (the facets) and the lateral intervertebral joints (the unco-vertebral joints). Soft tissue components, such as ligaments and tendons, constrain the overall segmental motion as well.
Traumatic, genetic, and long term wearing phenomena contribute to the degeneration of the nucleus in the human spine. This degeneration of this critical disc material, from the hydrated, elastomeric material that supports the separation and flexibility of the vertebral bones, to a flattened and inflexible state, has profound effects on the mobility (instability and limited ranges of appropriate motion) of the segment, and can cause significant pain to the individual suffering from the condition. Although the specific causes of pain in patients suffering from degenerative disc disease of the cervical spine have not been definitively established, it has been recognized that pain may be the result of neurological implications (nerve fibers being compressed) and/or the subsequent degeneration of the surrounding tissues (the arthritic degeneration of the facet joints) as a result of their being overloaded.
Traditionally, the treatment of choice for physicians caring for patients who suffer from significant degeneration of the cervical intervertebral disc is to remove some, or all, of the damaged disc. In instances in which a sufficient portion of the intervertebral disc material is removed, or in which much of the necessary spacing between the vertebrae has been lost (significant subsidence), restoration of the intervertebral separation is required.
Unfortunately, until the advent of spine arthroplasty devices, the only methods known to surgeons to maintain the necessary disc height necessitated the immobilization of the segment. Immobilization is generally achieved by attaching metal plates to the anterior or posterior elements of the cervical spine, and the insertion of some osteoconductive material (autograft, allograft, or other porous material) between the adjacent vertebrae of the segment. This immobilization and insertion of osteoconductive material has been utilized in pursuit of a fusion of the bones, which is a procedure carried out on tens of thousands of pain suffering patients per year.
This sacrifice of mobility at the immobilized, or fused, segment, however, is not without consequences. It was traditionally held that the patient's surrounding joint segments would accommodate any additional articulation demanded of them during normal motion by virtue of the fused segment's immobility. While this is true over the short-term (provided only one, or at most two, segments have been fused), the effects of this increased range of articulation demanded of these adjacent segments has become a concern. Specifically, an increase in the frequency of returning patients who suffer from degeneration at adjacent levels has been reported.
Whether this increase in adjacent level deterioration is truly associated with rigid fusion, or if it is simply a matter of the individual patient's predisposition to degeneration is unknown. Either way, however, it is clear that a progressive fusion of a long sequence of vertebrae is undesirable from the perspective of the patient's quality of life as well as from the perspective of pushing a patient to undergo multiple operative procedures.
While spine arthroplasty has been developing in theory over the past several decades, and has even seen a number of early attempts in the lumbar spine show promising results, it is only recently that arthoplasty of the spine has become a truly realizable promise. The field of spine arthroplasty has several classes of devices. The most popular among these are: (a) the nucleus replacements, which are characterized by a flexible container filled with an elastomeric material that can mimic the healthy nucleus; and (b) the total disc replacements, which are designed with rigid endplates which house a mechanical articulating structure that attempts to mimic and promote the healthy segmental motion.
Among these solutions, the total disc replacements have begun to be regarded as the most probable long-term treatments for patients having moderate to severe lumbar disc degeneration. In the cervical spine, it is likely that these mechanical solutions will also become the treatment of choice. At present, there are at least two devices being tested clinically in humans for the indication of cervical disc degeneration. The first of these is the Bryan disc, disclosed in part in U.S. Pat. No. 6,001,130. The Bryan disc is comprised of a resilient nucleus body disposed in between concaval-covex upper and lower elements that retain the nucleus between adjacent vertebral bodies in the spine. The concaval-convex elements are L-shaped supports that have anterior wings that accept bones screws for securing to the adjacent vertebral bodies.
The second of these devices being clinically tested is the Bristol disc, disclosed substantially in U.S. Pat. No. 6,113,637. The Bristol disc is comprised of two L-shaped elements, with corresponding ones of the legs of each element being interposed between the vertebrae and in opposition to one another. The other of the two legs are disposed outside of the intervertebral space and include screw holes through which the elements may be secured to the corresponding vertebra; the superior element being secured to the upper vertebral body and the inferior element being attached to the lower vertebral body. The opposing portions of each of the elements comprise the articulating surfaces that include an elliptical channel formed in the lower element and a convex hemispherical structure disposed in the channel.
Further improvements include U.S. Pat. No. 5,534,029 to Shima, which discloses an articulated vertebral body spacer including a pair of upper and lower joint pieces inserted between opposing vertebrae. The lower joint piece includes a convex portion formed on a central portion of its upper surface and having a convex sliding contact surface, and a stopper surface surrounding the convex portion. The upper joint piece includes a concave portion formed on a central portion of its lower surface and having a concave sliding contact surface which is in sliding contact with the convex sliding contact surface, and an abutment surface that surrounds the concave portion and abuts against the stopper surface. A cavity for allowing the upper joint piece to pivot in response to movement of the opposing vertebral bodies is formed between the abutment surface and the stopper surface.
DE 3529761 discloses a prosthesis for an intervertebral disc including two plates 1 with a spacer disc 4 therebetween. The two plates 1 each have a concave center and a flat annular rim 2 with spikes 3. The disc spacer 4 has a convex center and a flat rim with an annular groove 6. The prosthesis is used for spanning the gap between opposing vertebral faces remaining firmly in place while permitting natural movement of the spine.
U.S. Pat. No. 4,997,432 to Keller discloses a prosthesis including two stop plates 3 and a sliding body 4 arranged therebetween. The outer surfaces of the stop plates 3 have an essentially planar surface 5 provided with tooth-like projection 6 that penetrate into the vertebral bodies to fix the stop plates 3 securely to the vertebral bodies 1. The opposite side surfaces of the stop plates 3 include essentially spherical-shell-shaped recesses 7. The sliding core 4 has a spherical-shell-shaped projections 8 corresponding to the spherical-shell-shaped recesses 7. The stop plates 3 are made of metal and the sliding body 4 is made of a synthetic material.
U.S. Pat. No. 5,562,738 to Boyd et al. discloses an implant device having an ellipsoidally-shaped ball and socket oriented so that their greatest lengths are disposed along a first axis transverse to the anterior and posterior ends and their shortest lengths are disposed along a second axis which is perpendicular to the first axis along surface. A first joint surface is sloped away from the socket while a second joint surface remains flat. The degree of slope determines the amount of relative rotation between joint surfaces, and the first joint surface is sloped to provide for up to 5° of lateral bending in either direction, up to 5° of extension and up to 5° of Flexion.
Finally, U.S. Pat. No. 5,556,431 to Buttner-Janz discloses an intervertebral disc endoprosthesis that is inserted between two vertebrae and has a bottom plate and a top plate that are connected to vertebral endplates. Referring to FIG. 1, the device includes prosthesis plates 1 and 2 and prosthesis core 3 cooperated via spherical surfaces 4. The core 3 has an edge rim 5 that limits its range of movement and insures, even under extreme conditions cohesion of the prosthesis. The endplate 6 of the prosthesis plates 1, 2 lie on the end surfaces of the vertebrae and are provided with teeth 7, which, under load, penetrate into the vertebrae and thus secure the prosthesis in situ. Bore holes 8 are arranged symmetrically on both side of the central plane, running from ventral to dorsal, of the vertebrae and in the area of the front edge of the prosthesis plates 1, 2 to receive bone screws 9.
In spite of the above-noted advances in the art, there remains a need for an improved vertebral body spacer having enhanced stabilization and bone fusion characteristics.
With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification.
Referring now to FIGS. 1 and 2, a prior art intervertebral body cage 30 includes a tubular metal body 32 having threads 34 on an external surface thereof. Each cage 30 is inserted transverse to the axis of the spine 36, into preformed cylindrical holes at the junction of adjacent vertebral bodies (in FIG. 2 the pair of cages 30 are inserted between the fifth lumbar vertebra (L5) and the top of the sacrum (S1)). The two cages 30 are generally inserted side by side with the external threading 34 tapping into the lower surface of the vertebral bone above (L5), and the upper surface of the vertebral bone (S1) below. The cages 30 include holes 38 through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior 40 of the cage 30 to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage. 1.
These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient's spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.
Thus, there remains a need for an intervertebral spacer that stabilizes the spine, which enables adjacent vertebrae to move relative to one another, that supports compressive loads and that permits normal motion and rotation of the spinal segments.